For lift fans or blowers, is a thin plastic sheet covering the entire vehicle bottom (such as skirting) considered sufficient for the dowel test, or must there be additional "hard" guarding as there would be without the plastic sheet?

We can't prejudge specific devices. The dowel cannot contact the fans in an unsafe manner - if the vehicle bottom is thin enough to cause the dowel to push it into the fan and create a safety hazard then it would not be adequate and not be legal.